The state of a surface determines the quality of the thrust of a body thereagainst, in particular with respect to sealing effected between the surface and the body.
There is a discipline in metrology which deals specifically with surface states. This discipline may be referred to as "profilometry," and its purpose is to record, along a direction of the surface, the profile of its shape and of its irregularities referred to as "undulations" and "roughness", in compliance with French Standard No. E-05-015. Among the apparatuses (profilometers) used to effect these recordings, certain apparatuses have a contact tracer (a point which is more or less fine), and others use a ray of coherent light with the return signal therefrom being interpreted by interferometry.
The results of these recordings are stored in memories either graphically or in digital form and they constitute the closest possible image of the real profiles. By processing the step and the depths of the irregularities, the parameters are obtained concerning the profiles measured.
By calculation, and as a function of the recording methods, standardized parameter values are reached.
By taking a serie of recordings of profiles which are parallel and close together, a three-dimensional image of the scanned surface area is found.
Profilometry requires apparatuses which are not adapted to monitoring the quality of the surface state of parts in a production shop, in particular because these apparatuses are sensitive to mechanical and acoustic vibrations.
There are other apparatuses which produce data concerning the state of a surface. Mention should be made of capacitance measuring apparatuses in which the part constitutes one plate of a capacitor, and of workshop apparatuses which are essentially pneumatic and based on the principle of measuring the sonic flow of air through leaks, one of which takes place between the part and the measuring orifice of the apparatus.
Capacitance measuring apparatuses are limited to being used on surfaces that conduct electricity, and they are particularly sensitive to free metal particles which may lie on the surface if it is not cleaned correctly. Furthermore, they are not sensitive to undulation (misshapenness) and they are not suitable for use by a robot.
Pneumatic apparatuses are awkward to use because it is difficult to localize the sensor on the surface. They are slow to produce data because of the pressure-measuring systems they use. Finally, the surface quality of the walls of the nozzle influences pressure measurements.
There is currently a need to monitor regularity of production concerning surface state quality and to effect this monitoring on the production site itself.
An object of the invention is to meet this need by proposing an apparatus which is capable of comparing the geometrical characteristics of the roughness and the undulations of a plurality of surfaces and thus of assessing the quality of a surface state relative to a reference surface state, and which has members that are adapted to being handled automatically to ensure the apparatus is applied correctly against a part to be inspected and that do not require any prior adjustment of the measuring system.